1. Field of the Invention
The present invention relates to method and apparatus of measuring axial vibration of a rotating disk.
2. Description of the Related Art
A disk device such as a disk drive installed in a PC rotates an optical disk e.g., a compact disk or a digital versatile disk placed on a turntable at high speed with a spindle motor. While rotating the disk, the disk device conducts tracking and focusing operation of an objective lens of an optical pickup to detect signals written on a recording layer of the rotating disk without errors.
The tracking and the focusing operation are adjusted based on tracking and focusing error signal, respectively, that are produced out of beams reflected from a recording layer. Such tracking and focusing operation ensure an optimal signal reading condition.
However, if a rotating disk at high speed by a spindle motor vibrates axially, it is difficult for a focusing servo mechanism to maintain an exact focus on a recording layer. Thus, in this situation, a focusing servo gain need to be adjusted to increase sensitivity of a focusing servo.
In the event that axial vibration of a disk is too severe to compensate through gain adjustment of a focusing servo, it may be impossible to reproduce the disk rotating at high speed. If determined to impossible, the disk rotation speed must be reduced or the disk must be ejected without unnecessary reproduction attempts.
For such a reason, it is needed to measure accurately how much a disk vibrates axially.